Epidemiology of sorghum anthracnose (Colletotrichum sublineolum) and leaf blight (Exserohilum turcicum) in Kenya

Abstract

A 3-year study on the epidemiology of sorghum anthracnose (Colletotrichum sublineolum) and leaf blight (Exserohilum turcicum [Setosphaeria turcica]) was conducted at Alupe in western Kenya from 1994 to 1996. A nonlinear logistic model was used to summarize 72 anthracnose and 108 leaf blight disease progress curves from different planting dates and cultivars. Effects of planting date and cultivar on disease development were compared based on estimates of the rate (β), the absolute rate (θ), inflection point (μ) and upper asymptote (γ), disease severity at milk stage (v95), and time taken to reach a disease severity of 2% (t2, an estimate of time when disease is first observed). Leaf blight epidemics always started earlier than those of anthracnose, but exhibited lower disease severity at crop maturity. Effects of planting date and cultivar on β varied between years. Delaying planting reduced time to disease onset (i.e. µ and t2) and increased absolute rate of progress θ, resulting in maximum severity at crop 'milk stage' and maturity (v95 and γ, respectively). Resistant cultivars had highest µ and t2 but lowest θ, v95 and γ values for both diseases. Delaying planting affected anthracnose progress more on the resistant cultivars, in contrast with leaf blight for which progress on susceptible cultivars was more affected. The implications of these findings for disease management are discussed, and recommendations made to improve disease screening methodologies